This invention relates generally to cell encapsulation devices, and more particularly to devices for cell screening and implantation into a mammalian host.
In encapsulated cell therapy, xenogenic or allogenic cells are isolated from the host""s immune system by being surrounded in a semi-permeable membrane prior to implantation within the host. The semi-permeable membrane is relatively impermeable to large molecules, such as components of the host""s immune system, but is permeable to small molecules. Thus, the semi-permeable membrane allows the implanted cells to receive nutrients necessary for viability and allows metabolic waste to be removed. The membrane also allows therapeutic molecules produced by the implanted cells to diffuse to host cells. For example, endogenous proteins or those cloned into the cell are delivered to the host. The use of an immuno-protective, semi-permeable membrane now allows transplantation of encapsulated cells from one species into a host from a different species without the risk of immune rejection or use of immunosuppressive drugs. Applications of encapsulated cell therapy include, for example, treatment for diabetes, haemophilia, anemia, xcex2-thalassemia, Parkinson""s disease, and amyotropic lateral sclerosis.
The use of biologically compatible polymeric materials in construction of an encapsulation device is critical to a successful cell encapsulation therapy. Important components of the encapsulation device include the surrounding semi-permeable membrane and the internal cell-supporting matrix or scaffold. The scaffold defines the microenviromnent for the encapsulated cells and keeps the cells well distributed within the intracapsular compartment. The optimal internal scaffold for a particular cell encapsulation device is highly dependent on the cell type. For example, while adherent cells often prefer a solid surface on which to lie, suspension cells may prefer a hydrophilic lightly cross-linked hydrogel as a matrix material.
In the absence of a scaffold, adherent cells aggregate to form clusters. When the clusters grow too large, they typically develop a central necrotic core. Dying cells accumulate around the core and, upon lysing, release factors detrimental to the health of neighboring cells. The lysed cell fragments are also transported to the host environment, there eliciting an antigenic response.
Several types of prior art devices have attempted to solve these problems, meeting with mixed results. For example, the prior art includes the use of bonded fiber structures for cell implantation (U.S. Pat. No. 5,512,600) and the use of biodegradable polymers as scaffolds for organ regeneration such as, for example, liver, pancreas, and cartilage. The use of biodegradable polymers for use as scaffolds in organ regeneration is reviewed by Cima et al., BIOTECH. BIOENG. 38: 145-58 (1991). In these prior art works, biodegradable fiber tassels and fiber-based felts (i.e., non-woven materials) were used as scaffolds for transplanted cells. One drawback to the use of biodegradable polymers, particularly polymers of poly(lactic acid) PLA, poly(lactic-coglycolic acid) PLGA, poly(glycolic acid) PGA, and their equivalents, is that upon degradation, they release lactic and/or glycolic acid, which are toxic to surrounding tissue. As the polymers degrade, they break down to first low molecular weight oligomers and then to the acids, causing a rapid increase in acid released into the surrounding tissue. This rise in acid concentration in vivo in the local environment of the implant can induce an inflammatory response or tissue necrosis.
Foam scaffolds have also been used in the art to provide surfaces onto which transplanted cells may adhere. Foam scaffolds, however, have random flat surfaces and do not provide a linear template for reorganization. Some cell types prefer such a template for organization into physiological three-dimensional orientation.
Prior art also includes woven mesh tubes used as vascular grafts. Although cells may be seeded onto these woven tubes for improved biocompatibility, these tubes function primarily as vascular conduits and not cell scaffolds.
Thus, a need exists in the art for a non-degradable scaffold or framework system to provide an ordered linear environment for cells which prefer such an environment to grow and proliferate within cell encapsulation devices.
The invention provides a cell encapsulation device for growing, maintaining, proliferating and/or differentiating viable cells. The device has (1) an internal filamentous cell-supporting matrix or scaffolding comprising a plurality of filaments, preferably spun into one or more yarns, or alternatively woven into one or more mesh components, and (2) an encapsulating permselective membrane. Cells are seeded onto the yarn or mesh scaffolding, which is encapsulated by the permselective membrane. The cell matrix, or scaffold, in the device of the invention advantageously provides cells with a template for cellular organization in a three-dimensional orientation resembling their typical physiological shape. The cell matrix is particularly useful for the use of adherent cells in encapsulated cell therapy.
In one embodiment, the filamentous cell-supporting matrix, or scaffold, is made from any substantially non-degradable, biocompatible material. For example, the material can be acrylic, polyester, polyethylene, polypropylene, polyacetonitrile, polyethylene terephthalate, nylon, polyamides, polyurethanes, polybutester, silk, cotton, chitin, carbon, or biocompatible metals.
In a preferred embodiment, the core scaffold contains a plurality of monofilaments. In one example, the monofilaments are twisted into yarn. In another example, the plurality of monofilaments or the yarn is woven into mesh. In still other examples, the mesh is configured as a hollow cylinder, or tube, or woven into a solid cylinder, or cord.
In another embodiment, the scaffolding is coated with extracellular matrix (ECM) molecules. Suitable ECM molecules include, for example, collagen, laminin, and fibronectin. The surface of the scaffolding may also be modified by treating with plasma irradiation to enhance cell adhesion to the scaffolding.
The present invention provides several advantages over the prior art. Meshes and yarns have configurations that provide an ordered environment for cell types that may prefer such a template for reorganization into their physiological three-dimensional orientation. Also, the fibers used in the invention are substantially non-degradable and so do not release by-products into the host. Moreover, the yarn and mesh matrices of the invention have the following advantages over prior art hydrogel matrices: (1) they provide considerable added elasticity, compressive and tensile strength to hollow fiber membrane-based devices if attached at both ends of the device; (2) they provide a physical surface onto which extracellular matrix molecules may be attached; (3) they allow adherent cell types to attach and lay down their own extracellular matrix material; (4) they can be treated with a surface charge to enhance cell adhesion to the surface; (5) the yarn or mesh matrix can keep cells distributed more evenly both longitudinally and transversely and thus prevent cell clumping which leads to subsequent necrotic core formation; (6) they offer greater biological stability than hydrogel materials and have a long history of implant use as vascular grafts and suture materials; and (7) they allow certain cell types to orient in the direction parallel to the filaments or allow cells to bridge between filaments and form an ordered orientation.
In embodiments where the mesh woven tubes or yarns are affixed at both ends of the device, the overall device tensile strength, compressive strength, and kink resistance is also greatly improved. As a result, this insert of a mesh woven tube or yarns provides a method of inner support to strengthen the hollow fiber membrane device.